This invention relates to the technical field of circuits, and in particular, to a balun.
In general, a RF power amplifier usually uses a push-pull circuit. FIG. 1 shows a typical push-pull circuit. In the circuit, a balun (also known as balanced feeding converter or transmission line balancing device) is an indispensable component, wherein the input balun 1 is used to convert the signal on the unbalanced transmission line into a balanced input for the power amplifier, while the output balun 2 is used to convert the balanced output of the power amplifier onto an unbalanced load.
Above a S wave band, a balun can be implemented by using a ¼ wavelength transmission line easily, but for a UHF, VHF and even lower wave bands, because of longer wavelength, and limited by size, it can hardly be realized in this manner. The practice in the prior art is using ferrite material to realize the balun.
Some RF power amplifiers need to be used in a magnetic field environment, e.g., the RF power amplifier on a nuclear magnetic resonance (MR) device. The balun containing ferrite material cannot be used in a magnetic field environment, because the ferrite material may cause magnetic saturation.
The nuclear magnetic resonance device also has a pretty high requirement on the size and power capacity of the balun.
U.S. Pat. No. 3,976,959 discloses a balun, which comprises a flat insulating component, and a ground plane constructed by a conductor in the center of the insulating component, the conductor divides the insulating component into three parts, and further divides the received waveguide energy into two equiphase parts. The conductor can also recombine the signals of the two equiphase parts, and generate an output waveguide signal by a predefined phase relationship. In concrete implementation, micro strips and strip lines are employed. Its shortcoming is that the size will grow when used at a low frequency range, so is not applicable to the low frequency range application.
U.S. Pat. No. 6,531,943 discloses another structure of balun, which specifically comprises two structures. The first structure is using a coplanar waveguide and a coplanar strip line to realize the conversion between the balanced end and the unbalanced end, and this structure can be used in application scenarios from several MHz to 10 GHz: the second structure is a simplification based on the first one, using only a coplanar strip line, the node of an input end of the strip line is connected to ground, and the signal of its output end keeps a 180 degrees of phase difference, in addition, it further comprises a impedance matching circuit. This structure can be used in application scenarios from several MHz to 2 GHz. These two structures both have the same shortcoming; which include lower power capacity, and are not applicable to high power situations.